Firefighters have long been exposed to severe injuries from explosively propelled fire hose fittings. Fire trucks and stationary fire protection systems have many hose and adapter connection points that use fire hose thread for engagement. Trapped pressure is a constant hazard in this system, particularly when a firefighter tries to unscrew a connection. The trapped pressure can store a great deal of energy that, when released, can propel the fitting. This scenario has resulted in serious injury in the past. This situation is exacerbated when the trapped pressure includes a compressible gas like air.
The fire hose connection points use a face seal gasket to provide for a water-tight joint. When the threads are unscrewed with pressure in the system, the joint will leak slowly, but still retain pressure for an extended period of time due to the fact that the trapped fluid must travel through a narrow leak-path defined by the space between the male and female threads. If there is only compressed air in the system, there is no visual indication of the pressure. If water is present, the small discharge of water during this process is often even less at higher pressures than at lower pressures due to the fact that the pressure provides a tighter seal between the two surfaces of the male and female threads. This can lead the firefighter into believing that it is safe to uncouple the connection. The firefighter may not hear the louder “hissing” when the system is leaking at high pressure due to the noisy environment at the fire scene and the equipment worn by firefighters. The noise, therefore, which is usually the only indication of high pressure and a potentially dangerous situation, is an inadequate warning signal for firefighters.